Brussels

It seems that there is no 'exceptional' category in Brussels.
P : Protection (for old and/or partially destroyed buildings)
R : Restauration (For old building in good shape that must be kept as is)
S : Site - Zone
SA : Site archéologique - Archeological site

Wales

United States

The National Register of Historic Places (NRHP) is the United States federal government's official list of districts, sites, buildings, structures, and objects deemed worthy of preservation. These are generally of significance on any level, from national, to statewide, all the way down to the local level.

Criterion A, "Event," the property must make a contribution to the major pattern of American history.

Criterion B, "Person," is associated with significant people of the American past.

Criterion C, "Architecture/Engineering," concerns the distinctive characteristics of the building by its architecture and construction, including having great artistic value or being the work of a master.

Criterion D, "Information potential," is satisfied if the property has yielded or may be likely to yield information important to prehistory or history.

The criteria which apply to each site can be found in it's entry in the register.

A National Historic Landmark is a building, district, object, site, or structure that is officially recognized by the United States government for its outstanding historical significance. Of over 90,000 places listed on the country's National Register of Historic Places, only some 2,500 are recognized as National Historic Landmarks. In general, these are of significance on a national level and will be included in the National Register of Historic Places.

Texas Historical Commission

The Texas Historical Commission is an agency dedicated to historic preservation within the state of Texas. The commission identifies Recorded Texas Historic Landmarks and recognizes them with Official Texas Historical Marker medallions and descriptive plaques, State Archeological Landmarks and Historic Texas Cemeteries. The commission also operates the over 13,000 historical markers throughout the state of Texas.

The agency also maintains the online Texas Historic Sites Atlas featuring more than 300,000 site records, including data on Official Texas Historical Markers and National Register of Historic Places properties in Texas.

Recorded Texas Historic Landmark

Recorded Texas Historic Landmark (RTHL) is a legal designation and the highest honor the state can bestow on a historic structure. The Texas Historical Commission awards RTHL designation to buildings that are judged worthy of preservation based on architectural and historical merit.

The purpose of Official Texas Historical Markers, which are available in a variety of types (cemetery, building, subject) and sizes, is to educate the public about the history of the world around them. The reference numbers are all available on the Historic Sites Atlas linked above.